Add get_min_max() to Perspective API

[texodus]

Adds get_min_max() function to View objects in Python and Javascript, the purpose of which is to provide reasonable value ranges for rendering gradient-maps. get_min_max() takes a column name as an argument, and returns a tuple [min, max] of this column's View-type. This new version of min/max calculation works unlike the previous incarnation:

• It is a one-time-query, and does not recalculate unless called. The new <perspective-column-style> component calls this once only on instantiation.
• It does not include parent aggregates, e.g. anything with row_path or column_path less than their respective maxes, unless ...
• ... in the specific case of Row Pivots, when all children of a row depth are in a "collapsed" state, there are no non-parent aggregates, so the next non-empty depth level is used instead.

[texodus]

As @finos/perspective-viewer now depends on the presence of this call to open the new Column Style menu feature, and thus depends on a @finos/perspective or perspective-python version which supports this call, this feature should eb breaking